Method for producing 225Ac

ABSTRACT

A method for producing 225A including: a method (X) for purifying a 226Ra-containing solution, including an adsorption step of allowing a 226Ra ion to adsorb onto a carrier having a function of selectively adsorbing a divalent cation by bringing a 226Ra-containing solution into contact with the carrier under an alkaline condition, and an elution step of eluting the 226Ra ion from the carrier under an acidic condition; a method for producing a 226Ra target, including an electrodeposition liquid preparation step of preparing an electrodeposition liquid by using a purified 226Ra-containing solution obtained by the method (X), and an electrodeposition step of electrodepositing a 226Ra-containing substance on a substrate by using the electrodeposition liquid; and a step of irradiating a 226Ra target produced by the method for producing a 226Ra target with at least one selected from a charged particle, a photon, and a neutron by using an accelerator.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/622,390, filed on Dec. 23, 2021, which is a national stageapplication of PCT/JP2020/025059, filed on Jun. 25, 2020, and whichclaims priority to Japanese Patent Application No. 2019-123673, filed onJul. 2, 2019, the entire contents of all of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a method for purifying a²²⁶Ra-containing solution, a method for producing a ²²⁶Ra target, and amethod for producing ²²⁵Ac.

BACKGROUND ART

In the field of nuclear medicine, radionuclide therapy has beenperformed in which a drug containing a radioisotope (RI) is selectivelytaken into a lesion such as a tumor for treatment. Among radiations, analpha-ray has a characteristic that the effect of unnecessary exposureon the surrounding normal cells is small because the range is short.²²⁵Ac being one of the alpha-ray emitting nuclides is a radionuclidewith a half-life period of 10 days, and has been expected as atherapeutic nuclide in cancer treatment in recent years.

²²⁵AC is produced by a nuclear reaction of (p, 2n), for example, byirradiating a ²²⁶Ra target with a proton using an accelerator. PatentLiterature 1 discloses a method for separation and purification of an²²⁵AC component from a solution containing ²²⁶Ra ions and ²²⁵AC ions,which is obtained by dissolving the ²²⁶Ra target after irradiation.

CITATION LIST Patent Literature

Patent Literature 1: JP 2009-527731 A

SUMMARY OF INVENTION

However, there has been a problem that the amount of the ²²⁵AC producedfrom a ²²⁶Ra target is extremely small, and most of the ²²⁶Ra remainsunreacted.

Further, since ²²⁶Ra is a precious nuclide and the disposal of ²²⁶Ra isnot easy, it has been demanded to establish a method for purifyingefficiently and easily a solution containing ²²⁶Ra ions after ²²⁵ACseparation in order to reuse the unreacted ²²⁶Ra. The techniquedisclosed in Patent Literature 1 has a problem in terms of efficiencyand ease, such as the need for distillation and reflux.

The present invention has been made in view of the circumstances asdescribed above, and an object of the present invention is to provide amethod for purifying efficiently and easily a ²²⁶Ra-containing solutionobtained when ²²⁵Ac is produced from a ²²⁶Ra target, a method forproducing a ²²⁶Ra target by using the purified ²²⁶Ra-containing solutionobtained by the above purification method, and a method for producing²²⁵Ac including these above methods.

One embodiment of the present invention is a method for purifying a²²⁶Ra-containing solution, comprising the steps: (R1) of allowing a²²⁶Ra ion to adsorb onto a carrier having a function of selectivelyadsorbing a divalent cation by bringing a ²²⁶Ra-containing solution (a)into contact with the carrier under an alkaline condition; and (R2) ofeluting the ²²⁶Ra ions from the carrier under an acidic condition.

Further, another embodiment of the present invention is a method forproducing a ²²⁶Ra target, comprising the steps: (R4) of preparing anelectrodeposition liquid by using a purified ²²⁶Ra-containing solution(b) obtained by the method for purifying a ²²⁶Ra-containing solutiondescribed above; and (R5) of electrodepositing a ²²⁶Ra-containingsubstance on a substrate by using the electrodeposition liquid.

Furthermore, another embodiment of the present invention is a method forproducing ²²⁵Ac, comprising a step (A1) of irradiating a ²²⁶Ra targetproduced by the method for producing a ²²⁶Ra target described above withat least one kind selected from a charged particle, a photon, and aneutron by using an accelerator to produce ²²⁵Ac.

According to the method for purifying a ²²⁶Ra-containing solution of thepresent invention, a ²²⁶Ra-containing solution obtained when ²²⁵Ac isproduced from a ²²⁶Ra target can be purified efficiently and easily.Further, a ²²⁶Ra target can be produced efficiently by using thepurified ²²⁶Ra-containing solution obtained by the above purificationmethod. Furthermore, ²²⁵Ac can be obtained efficiently and stably by amethod for producing ²²⁵Ac including these above methods.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing an outline of the method for purifying a²²⁶Ra-containing solution, method for producing a ²²⁶Ra target, andmethod for producing ²²⁵Ac according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the present invention will be describedin detail. A flow chart showing an outline of the method for purifying a²²⁶Ra-containing solution, method for producing a ²²⁶Ra target, andmethod for producing ²²⁵Ac according to the present invention is shownin FIG. 1 .

Method for Purifying ²²⁶Ra-Containing Solution The method for purifyinga ²²⁶Ra-containing solution (hereinafter, also referred to as“purification method (X)”) according to the present invention ischaracterized by including: an adsorption step (R1) of allowing a ²²⁶Raion to adsorb onto a carrier having a function of selectively adsorbinga divalent cation (hereinafter, also referred to as “carrier (i)”) bybringing a ²²⁶Ra-containing solution (a) into contact with the carrier(i) under an alkaline condition; and an elution step (R2) of eluting the²²⁶Ra ions from the carrier (i) under an acidic condition. In this way,²²⁶Ra ions are concentrated, and impurities can be reduced. The solutionobtained by the purification method (X) is referred to as a purified²²⁶Ra-containing solution (b).

The ²²⁶Ra-containing solution (a) is not particularly limited as long asit is a solution containing ²²⁶Ra ions, and is preferably an aqueoussolution containing ²²⁶Ra ions. In order to perform an adsorption step(R1) under an alkaline condition, the ²²⁶Ra-containing solution (a) ispreferably an alkaline aqueous solution, and has a pH of preferably 8 ormore, and more preferably 9 or more. Examples of the alkaline aqueoussolution include an aqueous ammonium solution, an aqueous sodiumhydroxide solution, and an aqueous potassium hydroxide solution. In thisway, ²²⁶Ra ions can be adsorbed onto a carrier (i) efficiently.

As the ²²⁶Ra-containing solution (a), a solution after an irradiationstep (A1), a dissolution step (A2), and a separation step (A3) in theproduction method for producing ²²⁵Ac to be described later, that is, asolution obtained by separating an ²²⁵Ac component from a solution inwhich a ²²⁶Ra target irradiated with at least one kind selected from acharged particle, a photon, and a neutron by using an accelerator hasbeen dissolved may be used.

Adsorption Step (R1)

In an adsorption step (R1), ²²⁶Ra ions are adsorbed onto a carrier (i)by bringing a ²²⁶Ra-containing solution (a) into contact with thecarrier (i) under an alkaline condition.

The carrier (i) is not particularly limited as long as it can form acomplex with a metal ion under an alkaline condition and elute the metalion under an acidic condition. As the carrier (i), for example, acarrier having a divalent cation-exchange group can be mentioned. As thedivalent cation-exchange group, specifically, an iminodiacetic acidgroup, a polyamine group, or a methyl glycan group can be mentioned. Asthe divalent cation-exchange group, an iminodiacetic acid group ispreferable.

The carrier having a divalent cation-exchange group is not particularlylimited as long as the divalent cation-exchange group is retained on asolid-phase carrier such as a resin. A more preferable example of thecarrier includes a styrene-divinylbenzene copolymer retaining animinodiacetic acid group. Examples of the commercially available resinhaving an iminodiacetic acid group include the “Chelex” seriesmanufactured by Bio-Rad Laboratories, Inc., “DIAION” series manufacturedby Mitsubishi Chemical Corporation, and “Amberlite” series manufacturedby The Dow Chemical Company, and more specifically include “Chelex 100”(particle diameter: 50 to 100 mesh, and ionic form: Na form, Fe form)manufactured by Bio-Rad Laboratories, Inc. The carrier (i) may becharged in a tube for use. The tube is not particularly limited as longas the carrier (i) can be charged in the tube and the tube hasflexibility, and is preferably a flexible tube made of rubber, a resin,or the like, and more preferably a tube for medical use.

By using such a tube, a length longer than that of a common glass columncan be obtained, that is, the number of theoretical plates can beincreased, so that the adsorption efficiency of ²²⁶Ra ions can beincreased. Further, the carrier (i) through which a radioactivesubstance (²²⁶R-containing solution) has been passed can be easilydiscarded while being charged in a tube without radioactivelycontaminating other instruments, devices, and the like.

Elution Step (R2)

In an elution step (R2), ²²⁶Ra ions are eluted from a carrier (i) underan acidic condition. Specifically, by passing an inorganic acid throughthe carrier (i), the ²²⁶Ra ions adsorbed onto the carrier (i) can beeluted.

The inorganic acid is not particularly limited as long as it candissolve a ²²⁶Ra component adsorbed onto the carrier (i) and generateions, and examples of the inorganic acid include hydrochloric acid andnitric acid.

In this regard, from the viewpoints that ²²⁶Ra ions can be efficientlyeluted from the carrier and that anions derived from an inorganic acidcan be efficiently removed in the later step, the concentration of theinorganic acid is preferably 0.1 to 12 mol/L, more preferably 0.3 to 5mol/L, furthermore preferably 0.5 to 2 mol/L, and particularlypreferably 0.7 to 1.5 mol/L.

Anion Exchange Step (R3)

The purification method (X) according to the present invention mayfurther include an anion exchange step (R3) in which a solutioncontaining ²²⁶Ra ions eluted in an elution step (R3) is passed throughan anion exchange resin.

If any anions (for example, chloride ions or the like) derived from aninorganic acid (for example, hydrochloric acid or the like) used in theelution step (R2) remain in the solution, such anions may affect theelectrodeposition rate of ²²⁶Ra ions in an electrodeposition step (R5)described later. For this reason, it is preferable to treat the solutioncontaining the ²²⁶Ra ions eluted in the elution step (R2), in the anionexchange step (R3) because the anions derived from an inorganic acid canbe reduced by being exchanged for hydroxide ions, and theelectrodeposition efficiency of ²²⁶Ra ions in the electrodeposition step(R5) can be improved.

The anion exchange resin is not particularly limited as long as it canexchange anions (for example, chloride ions or the like) derived from aninorganic acid for hydroxide ions, and is preferably a strongly basicanion exchange resin, and more preferably a resin having a quaternaryammonium salt. Examples of the commercially available anion exchangeresin include the “MONOSPHERE” series manufactured by The Dow ChemicalCompany, and the “AG” series manufactured by Bio-Rad Laboratories, Inc.,and more specifically include “MONOSPHERE 550A” (particle diameter:590±50 mesh, ionic form: OH form).

In this regard, the anion exchange resin may be charged in a tube foruse in a similar manner as in the case of a carrier (i). As the tubecapable of being used for the charging, a tube similar to that in whichthe above-described carrier (i) is to be charged can be mentioned.

Other Step

A step of washing a carrier (i) may be included between the step (R1)and the step (R2) in a purification method (X). Specifically, a step canbe mentioned in which water is passed through a carrier (i). In thisway, the proportion of impurities contained in a purified²²⁶Ra-containing solution (b) can be reduced.

Method for Producing ²²⁶Ra Target The method for producing a ²²⁶Ratarget according to the present invention is characterized by includingan electrodeposition liquid preparation step (R4) of preparing anelectrodeposition liquid by using a purified 226Ra-containing solution(b) obtained by a purification method (X), and an electrodeposition step(R5) of electrodepositing a ²²⁶Ra-containing substance on a substrate byusing the electrodeposition liquid.

It is preferable that the method for producing a ²²⁶Ra target accordingto the present invention further includes a purification method(hereinafter, also referred to as “purification method (Y)”) includingan adsorption step (R6) of allowing ²²⁶Ra ions to adsorb onto a carrierhaving a function of selectively adsorbing divalent cations(hereinafter, also referred to as “carrier (ii)”) by bringing a²²⁶Ra-containing solution (c) after the electrodeposition step (R5) intocontact with the carrier (ii) under an alkaline condition, and anelution step (R7) of eluting the ²²⁶Ra ions from the carrier (ii) underan acidic condition. The solution obtained by the purification method(Y) is referred to as a purified ²²⁶Ra-containing solution (d).

Electrodeposition Liquid Preparation Step (R4)

In an electrodeposition liquid preparation step (R4), anelectrodeposition liquid is prepared by using a purified²²⁶Ra-containing solution (b), and at this time, a purified²²⁶Ra-containing solution (d) obtained by a purification method (Y) maybe mixed with the purified ²²⁶Ra-containing solution (b) to prepare anelectrodeposition liquid. In this way, the recovery rate of ²²⁶Ra can befurther increased, and ²²⁶Ra can be recovered more efficiently.

By adding as needed a buffer agent, an acid, or the like to a purified²²⁶Ra-containing solution (b) or a mixture of a purified²²⁶Ra-containing solution (b) and a purified ²²⁶Ra-containing solution(d), an electrodeposition liquid to be used in an electrodeposition step(R5) described later can be prepared.

Examples of the buffer agent include a chloride salt such as ammoniumchloride; a carbonate such as ammonium carbonate, sodium carbonate,potassium carbonate, calcium carbonate, or magnesium carbonate; ahydrogen carbonate such as ammonium hydrogen carbonate, sodium hydrogencarbonate, or potassium hydrogen carbonate; an acetate such as ammoniumacetate, sodium acetate, or potassium acetate; a succinate such asmonosodium succinate, disodium succinate, monopotassium succinate,dipotassium succinate, monoammonium succinate, or diammonium succinate;and a benzoate such as sodium benzoate, potassium benzoate, or ammoniumbenzoate. Among them, ammonium acetate is preferable from theviewpoints, for example, of being easy to maintain the pH of anelectrodeposition liquid within the desired range described later, andof electrodepositing ²²⁶Ra ions on a substrate more efficiently.

Examples of the acid include an inorganic acid, and a carboxylic acidhaving 2 to 6 carbon atoms. Examples of the inorganic acid includenitric acid, hydrochloric acid, and boric acid. Further, examples of thecarboxylic acid having 2 to 6 carbon atoms include acetic acid, succinicacid, and benzoic acid.

The acid is preferably a monovalent or divalent acid from the viewpointof improving the yield of ²²⁵Ac.

From the viewpoint that ²²⁶Ra ions can be more efficientlyelectrodeposited on a substrate, the pH of an electrodeposition liquidis preferably 4 to 7, and more preferably 5 to 6. The pH of theelectrodeposition liquid can be kept within the above range byappropriately adding a buffer agent or an acid.

The electrodeposition liquid may contain as needed a component that hasbeen used in conventional electroplating or the like within a range thatdoes not impair the effects of the present invention. As the othercomponents, one kind may be used, or two or more kinds may be used.

Electrodeposition Step (R5)

In an electrodeposition step (R5), a ²²⁶Ra-containing substance iselectrodeposited on a substrate by using an electrodeposition liquidprepared in an electrodeposition liquid preparation step (R4).

Examples of the ²²⁶Ra-containing substance include a ²²⁶Ra metal, and a²²⁶Ra salt. The obtained ²²⁶Ra target can be reused in an irradiationstep (A1) in a method for producing ²²⁵Ac described later.

Examples of the metal to be used for the substrate include aluminum,copper, titanium, silver, gold, iron, nickel, niobium, and alloyscontaining these metals (such as phosphor bronze, brass, nickel silver,beryllium copper, Corson alloy, and stainless steel).

Further, the substrate may be plated a conductive support with thesemetals.

As the substrate, a gold plate is preferable, for example, from theviewpoint of being less likely to cause adverse effects on anaccelerator and the like even during irradiation with at least one kindselected from a charged particle, a photon, and a neutron by using theaccelerator and of being capable of preventing contamination with ametal derived from the substrate during the irradiation or thedissolution of a target, and from the viewpoint of being capable ofelectrodepositing ²²⁶Ra ions on a substrate more efficiently.

The electrodeposition step (R5) can be performed by a known method.Specifically, by energizing an electrodeposition liquid, a²²⁶Ra-containing substance is electrodeposited on a substrate.

As the power source for energization, it is not particularly limited,and a direct current (DC) power source, an alternating current (AC)power source, a pulse power source, a PR pulse power source, or the likecan be used. Among them, a pulse power source or a PR pulse power sourceis preferably used, for example, from the viewpoints of being easy toimprove the diffusion of ²²⁶Ra ions and to uniformly electrodeposit a²²⁶Ra-containing substance, being capable of suppressing the generationof heat, and being capable of performing the electrodeposition with asmall power source.

As the temperature (temperature of electrodeposition liquid) in theelectrodeposition step (R5), it is not particularly limited, and atemperature of, for example, around 10 to 80° C. can be employed.

Adsorption Step (R6)

In an adsorption step (R6), ²²⁶Ra ions are allowed to adsorb onto acarrier (ii) by bringing a ²²⁶Ra-containing solution (c) containingresidual ²²⁶Ra ions after an electrodeposition step (R5) into contactwith the carrier (ii) under an alkaline condition.

As the carrier (ii), a carrier similar to the carrier (i) to be used inan adsorption step (R1) in a purification method (X) can be used, andthe carrier (ii) may be charged in a tube for use in a similar manner asin the case of the purification method (X).

Elution Step (R7)

In an elution step (R7), ²²⁶Ra ions are eluted from a carrier (ii) underan acidic condition. Specifically, by passing an inorganic acid throughthe carrier (ii), the ²²⁶Ra ions adsorbed onto the carrier (ii) can beeluted.

As the inorganic acid to be used in the elution step (R7), an inorganicacid similar to that to be used in an elution step (R2) can be used, andthe inorganic acid can also have a concentration similar to that of theinorganic acid to be used in the elution step (R2).

Anion Exchange Step (R8)

A purification method (Y) may further include an anion exchange step(R8) in which a solution containing ²²⁶Ra ions eluted in an elution step(R7) is passed through an anion exchange resin.

If any anions (for example, chloride ions or the like) derived from aninorganic acid (for example, hydrochloric acid or the like) used in theelution step (R7) remain in the solution, such anions may affect theelectrodeposition efficiency of ²²⁶Ra ions when an electrodepositionliquid is prepared in an electrodeposition liquid preparation step (R4)and then an electrodeposition step (R4) is performed. For this reason,it is preferable to treat a solution containing the ²²⁶Ra ions eluted inthe elution step (R7), in an anion exchange step (R8) because the anionsderived from an inorganic acid can be reduced by being exchanged forhydroxide ions, and the electrodeposition rate of the ²²⁶Ra ions can beimproved in a case where the solution is used again as anelectrodeposition liquid in the electrodeposition step (R4).

Other Step

A step of washing a carrier (ii) may be included between the step (R6)and the step (R7) in a purification method (Y). Specifically, a step canbe mentioned in which water is passed through a carrier (ii). In thisway, the proportion of impurities contained in a purified²²⁶Ra-containing solution (d) is reduced.

Method for Producing ²²⁵Ac

The method for producing ²²⁵Ac according to the present invention ischaracterized by including an irradiation step (A1) of irradiating a²²⁶Ra target produced by the above-described method for producing a²²⁶Ra target according to the present invention with at least one kindselected from a charged particle, a photon, and a neutron by using anaccelerator. It is preferable that the method for producing ²²⁵Acaccording to the present invention further includes a dissolution step(2) of dissolving the ²²⁶Ra target irradiated in the irradiation step(A1), and a separation step (A3) of separating a colloidal ²²⁵Accomponent by alkalizing the solution obtained in the dissolution step(A2).

Irradiation Step (A1)

In an irradiation step (A1), a ²²⁶Ra target produced by theabove-described method for producing a ²²⁶Ra target according to thepresent invention is irradiated with at least one kind selected from acharged particle, a photon, and a neutron by using an accelerator, and²²⁵Ac is allowed to generate by a nuclear reaction. As the particle, aproton, a deuteron, an a particle, or a γ particleis preferable, and aproton is more preferable.

In this regard, as for the irradiation method and the irradiationcondition, a known method and a known condition can be adopted.

Dissolution Step (A2)

In a dissolution step (A2), a ²²⁶Ra target irradiated in an irradiationstep (A1) is dissolved in an acid solution. As a result, a solutioncontaining ²²⁶Ra ions and ²²⁵Ac ions is obtained.

As the acid solution, an acid solution that can dissolve ²²⁵Ac and ²²⁶Raas ions is mentioned, and specifically an aqueous solution of aninorganic acid such as hydrochloric acid, or nitric acid, preferably anaqueous solution of hydrochloric acid is mentioned.

Separation Step (A3)

In a separation step (A3), a colloidal ²²⁵Ac component by alkalizing asolution obtained in a dissolution step (A2) is separated.

The ²²⁵Ac dissolved in water as ²²⁵Ac ions under an acidic conditionbecomes actinium hydroxide ²²⁵Ac (OH)₃) under an alkaline condition, andforms colloids in an aqueous solution. The colloidal actinium hydroxideis collected on a filter by filtering with a membrane filter or thelike, and can be separated from the solution.

In addition, a ²²⁶Ra component exists as ions in a solution to which analkaline solution has been added, and is separated from an ²²⁵Accomponent by a separation step (A3), and a ²²⁶Ra-containing solution (a)is obtained. The obtained ²²⁶Ra-containing solution (a) is supplied toan adsorption step (R1) in a purification method (X).

Recovery Step (A4)

By dissolving ²²⁵Ac separated in a separation step (A3) with an acidsolution, an ²²⁵Ac-containing solution is obtained. The obtained²²⁵Ac-containing solution may be further purified by a known method, asneeded.

Dissolution

Actinium hydroxide separated in a separation step (A3) can be dissolvedby using an acid solution. The acid solution to be used for dissolutionis not particularly limited as long as it can dissolve actiniumhydroxide as ions, and for example, the same acid solution as that usedin a dissolution step (A2) can be used. Further, it is preferable thatthe concentration is 1 to 6 mol/L, and more preferably 2 to 5 mol/L,from the viewpoints that actinium hydroxide is easily dissolved as ionsand that a carrier easily adsorbs ²²⁶Ra.

Purification

A solution containing ²²⁵Ac ions dissolved with an acid solution can bepurified, for example, by a solid-phase extraction method. A solid-phaseextraction agent to be used in the solid-phase extraction method is notparticularly limited as long as it can capture ²²⁵Ac ions and then elutethe ²²⁵Ac ions under a predetermined condition, and examples of thesolid-phase extraction agent include ones containing a compoundrepresented by the formula (1).

In the formula (1), m and n are independently 0 or 1, and preferably 1;and R¹, R², R³, and R⁴ are independently a straight or branched chainalkyl group having 8 or more and 12 or less carbon atoms, and preferablyindependently an octyl group or 2-ethylhexyl. Such a solid-phaseextraction agent is commercially available, for example, as “DGA Resin”manufactured by Eichrom Technologies Inc.

As the specific purification method, first, an ²²⁵Ac-containing solutionis passed through a solid-phase extraction agent to capture ²²⁵Ac ionsand the like in the solid-phase extraction agent. Next, the capturedunnecessary ²²⁶Ra is eluted by passing the solution through asolid-phase extraction agent with an inorganic acid such as hydrochloricacid. At this time, the concentration of the inorganic acid is set to arelatively high concentration so that ²²⁵Ac does not elute. After that,²²⁵Ac ions can be eluted from the solid-phase extraction agent bypassing through an inorganic acid having a relatively low concentration.

EXAMPLES

Hereinafter, the present invention is further specifically described onthe basis of Examples, however, the present invention is in no waylimited to these Examples.

Examples 1 and 2 Evaluation Item 1: Mass Balance of ²²⁶Ra inPurification Method (X)

An irradiated ²²⁶Ra target (size: 010 mm, thickness: 2 to 3 mm, and²²⁶Ra mass: 0.3 to 1 mg) was dissolved in 5 mL of 1 mol/L hydrochloricacid, and then the obtained solution was filtered with a membrane filterto remove insoluble matters. To the filtrate, 1 mL of 28% by massammonia water (product name: Ammonia solution (25.0 to 27.9%) for atomicabsorption spectrometry, manufactured by KANTO CHEMICAL CO., INC.) wasadded to adjust the pH to 10 to 12, and colloid of actinium hydroxidewas generated. Next, the generated actinium hydroxide was filtered byusing a membrane filter at a flow rate of 1 to 2 mL/min to recover a²²⁶Ra-containing solution (a-1). The radioactivity of the obtained²²⁶Ra-containing solution (a-1) was measured by a germaniumsemiconductor detector manufactured by EURISYS MESURES.

Next, in order to prevent contamination by Na in a purified²²⁶Ra-containing solution (b-1) described later, one that had beenobtained by converting Chelex 100 (particle diameter: 50 to 100 mesh,ionic form: Na form, and use amount: 3 mL, manufactured by Bio-RadLaboratories, Inc.) to a NH₄+ form was charged in a medical tube havingan inner diameter of 3.2 mm, an outer diameter of 4.4 mm, and a lengthof 50 cm (extension tube, 3.2×4.4×500 mm (4 mL), MS-FL, manufactured byHAKKO CO., LTD.), 50 to 80 mL of the obtained ²²⁶Ra-containing solution(a-1) (pH >9) was passed through the medical tube at a flow rate of 1 to2 mL/min, and the eluate was taken as a waste liquid (W1). Next, 10 mLof water was passed through Chelex 100 at a flow rate of 1 to 2 mL/min,and the eluate was merged with the waste liquid (W1).

Next, MONOSPHERE 550A (particle diameter: 590 ±50 mesh, ionic form: OHform, and use amount: 20 mL, manufactured by The Dow Chemical Company)was washed with hydrochloric acid, water, sodium hydroxide, and water inthis order, and then the washed MONOSPHERE 550A was charged in a medicaltube having an inner diameter of 3.2 mm, an outer diameter of 4.4 mm,and a length of 200 cm (extension tube, 3.2×4.4×500 mm (4 mL), MS-FL,manufactured by HAKKO CO., LTD.), and the medical tube was connected toa tube filled with Chelex 100. 10 mL of 1 mol/L hydrochloric acid waspassed through Chelex 100 and MONOSPHERE 550A in this order at a flowrate of 1 to 2 mL/min, and then 8 mL of water was further passed throughthe resultant Chelex 100 and MONOSPHERE 550A at a flow rate of 1 to 2mL/min, and 18 mL of a purified ²²⁶Ra-containing solution (b-1) wasobtained.

The radioactivity of the obtained purified ²²⁶Ra-containing solution(b-1) was measured by a germanium semiconductor detector. Further, theradioactivity of each of the waste liquid (W1) and the materials ofChelex and MONOSPHERE 550A was measured in order to examine thedistribution of residual ²²⁶Ra.

The same operation was performed twice in total (Examples 1 and 2), andthe mass balance of each ²²⁶Ra was calculated. The results are shown inTable 1.

TABLE 1 Example 1 Example 2 Numerical Numerical value Percentage valuePercentage ²²⁶Ra-Containing 9.13 MBq 100%  2.97 MBq 100%  solution (a-1)(calculated value) Waste liquid (W1) N.D <0.2%*¹ N.D <0.7%*¹ ResidualChelex after N.D <0.2%*¹ N.D <0.7%*¹ in elution material MONO- N.D<0.2%*¹ N.D <0.7%*¹ SPHERE 550A after elution Purified ²²⁶Ra-containing9.13 MBq >99.3%   2.97 MBq >98.0%   solution (b-1)

In Table 1, the ²²⁶Ra-containing solution (a-1) was calculated from thefollowing formula (1).

²²⁶Ra (calculation value) of ²²⁶Ra-containing solution (a-1)=purified²²⁶Ra-containing solution (b-1)+residual ²²⁶Ra in Chelex 100+residual²²⁶Ra in MONOSPHERE 550A+waste liquid (W1) . . . (1)

In this regard, for the purified ²²⁶Ra-containing solution (b-1) ofExample 2, a value calculated from the difference in radioactivitybetween the Ra-adsorbed Chelex and the Ra-eluted Chelex was used.

The values with *1 shown in Table 1 were calculated assuming that amaximum of 0.02 MBq was detected because it is unclear whether themeasurement of less than 0.02 MBq is possible although the measuredvalue was N.D.

As in Examples 1 and 2, by passing the ²²⁶Ra-containing solution (a-1)through Chelex 100, impurities (ammonium chloride (hydrochloricacid+ammonia), ammonia, and the like) other than ²²⁶Ra can be removed.Further, most of the chloride ions can be removed by these adsorptionstep (R1), elution step (R2), and anion exchange step (R3).

Examples 3 to 8 Evaluation Item 2: Mass Balance of ²²⁶Ra afterDissolution Step (A2) and Separation Step (A3)

An irradiated ²²⁶Ra target (size: Φ10 mm, thickness: 2 to 3 mm, and²²⁶Ra mass: 0.3 to 1 mg) was dissolved in 5 mL of 1 mol/L hydrochloricacid, and then the obtained solution was filtered with a membrane filterto remove insoluble matters. To the filtrate, 1 mL of 28% by massammonia water (product name: Ammonia solution (25.0 to 27.9%) for atomicabsorption spectrometry, manufactured by KANTO CHEMICAL CO., INC.) wasadded to adjust the pH to 10 to 12, and colloid of actinium hydroxidewas generated. Next, the generated actinium hydroxide was filtered byusing a membrane filter at a flow rate of 1 to 2 mL/min to obtain a²²⁶Ra-containing solution (a-2).

Next, DGA Resin (DGA Normal Resin, 1-mL cartridge, manufactured byEichrom Technologies Inc.) was connected to a membrane filter. 6 mL of 4mol/L nitric acid was passed through the membrane filter and the DGAResin in this order at a flow rate of 1 to 2 mL/min, and the eluate wastaken as a waste liquid (W2).

The radioactivity of the solution after the dissolution step (A2) wasmeasured by a germanium semiconductor detector. Further, theradioactivity of each of the waste liquid (W2) and the materials ofmembrane filter and DGA Resin was measured by a germanium semiconductordetector in order to examine the distribution of residual ²²⁶Ra. Thesame operation was performed three times in total (Examples 3 to 5), andthe mass balance of each ²²⁶Ra was calculated. The results are shown inTable 2.

Evaluation Item 3: Mass Balance of ²²⁵Ac

Next, the DGA Resin was removed from the membrane filter, 6 mL of 8mol/L hydrochloric acid was passed through the DGA Resin at a flow rateof 1 to 2 mL/min, and the eluate was taken as a waste liquid (W3). Afterthat, 10 mL of 0.01 mol/L hydrochloric acid was passed through the DGAResin at a flow rate of 1 to 2 mL/min, and an ²²⁵Ac-containing solutionwas obtained.

The radioactivity of the obtained ²²⁵Ac-containing solution was measuredby a germanium semiconductor detector. Further, the radioactivity ofeach of the waste liquid (W3) and the materials of membrane filter andDGA Resin was measured by a germanium semiconductor detector in order toexamine the distribution of residual ²²⁵Ac. The same operation wasperformed three times in total (Examples 6 to 8), and the results areshown in Table 3.

TABLE 2 Example 3 Example 4 Example 5 Numerical Numerical Numericalvalue Percentage value Percentage value Percentage Solution after 11.70MBq    100% 17.03 MBq    100% 35.90 MBq    100% dissolution step (A2)Residual Membrane filter N.D  <0.2%^(*1) N.D  <0.1%*¹  0.02 MBq    0.1%in DGA Resin N.D  <0.2%^(*1) N.D  <0.1%*¹ N.D  <0.1%^(*1) material Wasteliquid (W2)  0.18 MBq    1.5%  0.03 MBq    0.2%  0.46 MBq    1.3%Recovered ²²⁶Ra-Containing 11.52 MBq >98.1% 17.00 MBq >99.6% 35.43MBq >98.6% amount solution (a-2) not containing waste liquid (W2)(calculated value) ²²⁶Ra-Containing 11.70 MBq >98.7% 17.03 MBq >99.8%35.88 MBq >99.9% solution (a-2) containing waste liquid (W2) (calculatedvalue)

TABLE 3 Example 6 Example 7 Example 8 Numerical Numerical Numericalvalue Percentage value Percentage value Percentage Membranefilter-collected 49.30 kBq    100% 47.55 kBq    100% 73.45 kBq    100%amount (calculated value) Residual Membrane filter^(*2) N.D  <1.2%^(*1)N.D  <1.2%^(*1)  0.93 kBq   1.27% in DGA Resin^(*2) N.D  <1.2%^(*1) N.D <1.2%^(*1) N.D  <0.8%^(*1) material Waste liquid (W3) after N.D <1.2%^(*1)  1.67 kBq   3.50% N.D  <0.8%^(*1) passed through DGAResin^(*2) Recovered After membrane 49.30 kBq >96.5%^(*3) 45.88 kBq>94.1%^(*3) 72.51 kBq >97.1%^(*3) amount collection^(*2)

The values with *1 shown in Table 2 were calculated assuming that amaximum of 0.02 MBq was detected because it is unclear whether themeasurement of less than 0.02 MBq is possible although the measuredvalue was N.D.

The ²²⁶Ra-containing solution (a-2) not containing the waste liquid (W2)after the separation step (A3) in Table 2 was calculated from thefollowing formula (2).

²²⁶Ra-containing solution (a-2) not containing waste liquid (W2)(calculation value) =²²⁶Ra contained in the solution after dissolutionstep (A2)—residual ²²⁶Ra in the material after separation step (A3)(membrane filter)—residual ²²⁶Ra in the material after separation step(A3) (DGA Resin) —²²⁶Ra amount of the waste liquid (W2) after separationstep (A3) . . . (2)

The amount of ²²⁶Ra contained in the solution after the dissolution step(A2) was calculated by collecting a part of the solution, measuring theamount of the part, and converting the measured amount of the part to anamount for the entire solution.

The values with *1 shown in Table 3 were calculated assuming that amaximum of 0.58 kBq was detected because it is unclear whether themeasurement of less than 0.58 kBq is possible although the measuredvalue was N.D.

The values with *2 shown in Table 3 were used to calculate the ²²⁵Acmembrane filter-collected amount after the separation step (A3).

The values of the membrane filter-collected amount shown in Table 3 werecalculated from the following calculation formula (3).

Membrane filter-collected amount (calculation value) after separationstep (A3)=residual ²²⁵Ac in the material after ²²⁵Ac recovery step (A4)(membrane filter)+residual ²²⁵Ac in the material after ²²⁵Ac recoverystep (A4) (DGA Resin) +²²⁵Ac amount of the waste liquid (W3) after beingpassed through DGA Resin of ²²⁵Ac recovery step (A4) ²²⁵Ac recoveredamount in purified ²²⁵Ac-containing solution (3)

In this regard, as the ²²⁵Ac recovered amount in the purified²²⁵Ac-containing solution of Example 6, a value calculated from thedifference in radioactivity between the Ac adsorbed-DGA and theAc-eluted DGA was used.

The values with *3 shown in Table 3 are each the mass balance aftercollection by a membrane filter, and the uncollected matters by themembrane filter are not taken into consideration. Further, since theradioactivity of ²²⁵Ac in the ²²⁶Ra solutions before and after beingpassed through a membrane filter cannot be measured due to the influenceof ²²⁶Ra, the recovery rate was calculated by assuming the denominatoras “the membrane filter-collected amount after separation step (A3)(calculation value)”.

The invention claimed is:
 1. A method for producing ²²⁵Ac, comprising: amethod (X) for purifying a ²²⁶Ra-containing solution, comprising anadsorption step (R1) of allowing a ²²⁶Ra ion to adsorb onto a resincarrier having a function of selectively adsorbing a divalent cation bybringing a 226Ra-containing solution (a) into contact with the carrierunder an alkaline condition, and an elution step (R2) of eluting the²²⁶Ra ion from the carrier under an acidic condition; a method forproducing a ²²⁶Ra target, comprising an electrodeposition liquidpreparation step (R4) of preparing an electrodeposition liquid by usinga purified ²²⁶Ra-containing solution (b) obtained by the method (X), andan electrodeposition step (R5) of electrodepositing a ²²⁶Ra-containingsubstance on a substrate by using the electrodeposition liquid; and astep (A1) of irradiating a ²²⁶Ra target produced by the method forproducing a ²²⁶Ra target with at least one kind selected from a chargedparticle, a photon, and a neutron by using an accelerator to produce²²⁵Ac.
 2. The method for producing ²²⁵Ac according to claim 1, whereinthe carrier has a divalent cation-exchange group.
 3. The method forproducing ²²⁵Ac according to claim 1, wherein the carrier has animinodiacetic acid group.
 4. The method for producing ²²⁵Ac according toclaim 1, the method (X) further comprises a step (R3) of performinganion exchange by passing a solution containing a ²²⁶Ra ion eluted inthe elution step (R2) through an anion exchange resin.
 5. The method forproducing ²²⁵Ac according to claim 1, wherein the ²²⁶Ra-containingsolution (a) is obtained by separating an ²²⁵Ac component from asolution in which a ²²⁶Ra target irradiated with at least one kindselected from a charged particle, a photon, and a neutron by using anaccelerator has been dissolved.
 6. The method for producing ²²⁵Acaccording to claim 1, wherein the carrier is charged in a tube.
 7. Themethod for producing ²²⁵Ac according to claim 1, further comprising apurification method (Y) comprising the steps: (R6) of allowing a ²²⁶Raion to adsorb onto a carrier having a function of selectively adsorbinga divalent cation by bringing a ²²⁶Ra-containing solution (c) after theelectrodeposition step (R5) into contact with the carrier under analkaline condition; and (R7) of eluting the ²²⁶Ra ion from the carrierunder an acidic condition, wherein a purified ²²⁶Ra-containing solution(d) obtained by the purification method (Y) is mixed with the purified²²⁶Ra-containing solution (b), and an electrodeposition liquid isprepared in the electrodeposition liquid preparation step (R4).
 8. Themethod for producing ²²⁵Ac according to claim 7, the purification method(Y) further comprises a step (R8) of performing anion exchange bypassing a solution containing a ²²⁶Ra ion eluted in the elution step(R7) through an anion exchange resin.
 9. The method for producing ²²⁵Acaccording to claim 1, further comprising the steps: (A2) of dissolvingthe ²²⁶Ra target irradiated in the irradiation step (A1); and (A3) ofseparating a colloidal ²²⁵Ac component by alkalizing the solutionobtained in the dissolution step (A2).